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UNITED STATES OF AMERICA. 



A KEY TO 



Shaws Physics by Experiment 



EDWARD E. SHAW 

DEAN OF THE SCHOOL, OF PEDAGOGY, UNIVERSITY OF THE 
CITY OF NEW YORK 



3?7 






NEW YORK 
Mayxakd, Merrill, & Co., Publishers 

4^>, 45 and 47 East Tenth Street 
180-4 



Copyright, 1894, by 
MAYNABD, MERRILL, & ( <>. 



Press of J. J. Little & Co. 
Astor Place, New York 






A KEY 



SHAW'S PHYSICS BY EXPERIMENT. 

Xote.— Many of the questions in Physics by Experiment are simple, 
and their answers are self-evident. In this Key answers are given to 
all questions and experiments where there is any possibility of difficulty 
or doubt. 

CHAPTEE I. 
Experiment 1, pages 10, 11. 

Ten grams over the three-inch mark, balance five grams 
over the end mark. 

When twenty-five grams of weight are placed two inches 
to the left of the fulcrum, ten grams must be placed five 
inches to the right, to balance the lever. 

The weight remaining the same, twelve and one-half 
grams must be placed four inches to the right, to balance 
the lever. And so on. 

Experiment 2, page 11. 

When the fulcrum is under the three-inch mark, the Wd 
is three inches long, and the Pel is nine inches long. Ten 
grams on the end of the lever will now balance thirty grams 
weight. 

Proolems, page 12. 

1. P in lbs. x Pel in feet -4- ^Ycl in feet = W in lbs. 
150 x 5 -r- 1 = 750 

2. 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

P in lbs. x Pd in feet -=- Wd in feet = W in lbs. 
150 x V -*■ i = 1650 



4. 150 



= 1200 



Questions, page 17. 

1. The Wd is part of the Pd in (he Second Class of levers. 

2. When the thumb-latch is pressed down with the thumb, it repre- 
sents a lever of the First Class ; when lifted from the other side of the 
door, it represents a lever of the Second Class. 

3. The Pd is part of the Wd in the Third Class of levers. 

4. A pair of sugar-tongs belongs to the Third Class of levers ; a 
lemon-squeezer to the Second Class ; a pair of scissors to the First 
Class. 

5. Illustrations of levers of the First Class : the common pump- 
handle, a pair of wire-cutters, the key of a piano. Of the Second Class • 
a door, a desk-lid, a window-catch. Of the Third Class : a pair of 
sheep-shears, the safety-valve of a boiler, a pair of dividers when prop- 
erly opened. 

6. The weight is never less than the power in levers of the Second 
Class. The power is always more than the weight in levers of the 
Third Class. 

Problems, page 18. 

1. P in lbs. x Pd in ins. -f- Wd in ins. = W in lbs. 
250 x 48 - 8 1500 

It is a simple matter to draw these levers according to scale. 



2. W in lbs. x Wd in ins. 
950 x U 



Pd in ins. = P in lbs. 
51 = 167H 



3. P in lbs. x Pd in ins. - Wd in ins. = W in lbs 

225 x 39 - 9 = 075 

4. An oar is a lever of the Second Class. 

When the Wd is greater than the Pd, time is gained. When the Pd 
is greater than the Wd, power is gained. 

Experiment lO. 

While raising the stone, the spring-balance shows the 
application of more force than is required to suspend' the 



A KEY TO SHAWS PHYSICS BY EXPEEIMENT. 5 

stone at rest, because force is required to overcome both 
the inertia of the stone and the friction of the pulley. 

Tests, page 26. 

The screws on the ends of wagon and carriage axles are arranged 
as right-hand and left-hand screws, so that the motion of the wheel 
may tend to tighten rather than to unscrew the nut. 

The turn-buckle is one solid piece, consisting of two nuts, one at each 
end, joined by two bars. With every turn of the buckle the two rods 
move towards each other or from each other, according to the direction 
in which the turn-buckle revolves. 

Questions and Problems, pages 27-29. 

1. In the treadle of the sewing-machine levers of the First and Third 
Classes are involved. A lever of the First Class when pressure is made 
with the heel ;/ a lever of the Third Class when pressure is made with 
the toe. 

2. The product of the power by the power-distance equals the prod- 
uct of the weight by the weight-distance. 

3. The three important points of the lever are the fulcrum, the point 
where the weight is supported, and the point where the power is 
applied. 

4. The wheelbarrow is of the Second Class of levers. 

5. Fig. 25 represents a lever of the Second Class, Fig. 26 of the Second 
Class, Fig. 27 of the First Class. 

6. The fore-arm is a lever of the Third Class. 

7. W in lbs. x Wd in ins. -f- Pd in ins. = P in lbs. 

250 x 21 -f- 60 = 87| 

8. Considering the beam to be a lever of the Second Class and the 
support at one end the fulcrum, find the power required at the other 
end to sustain the weight, 250 lbs. Then regard the other support as 
the fulcrum and proceed as before. Add to each of these results half 
the weight of the beam. 

62| lbs. power + 24 lbs. beam = 864- lbs. pressure. 
187£ lbs. power + 24 lbs. beam = 21H lbs. pressure. 
9. A lever of the First Class is shown in Fig. 29. 

10. The claw-hammer is a lever of the First Class. 

11. 2000, Win lbs, -i- 8, no. of parts of cord supporting W, = 050, 
P in lbs. 



A KEY TO SHAW S PHYSICS BY EXPERIMENT. 



12. 


W in 


lbs. 


X 


Wd in 


ins. - 


- Pd 


in 


ins. 


= 


/' in Lbs. 




70 




X 




4 






13 




= 


31ft 


13. 


P in 


lbs. 


x 


Pd 


in 


ins. - 


- Wd 


in 


ins. 


= 


11' in lbs 




250 


x 2 


X 




30 




11 


0] 


5* 


= 


2727.27 



14. W x height of wagon (Wd) -=- length of plane (Pd) = Pin lbs. 
300 x 3 -=-7 = 1284 

15. Applications of the inclined plane are seen in skids, the tread- 
mill, a winding roadway on a hillside. 

16. IFin lbs. x Wd in ins. -~ Pd in ins. = Pin lbs. 

2000 x 1 -*-' 12 = 1661 

17. A force of six tons must be exerted at A. 

18. Pin lbs. x Pd in ins. -f- Wd in ins. = Win lbs. 

3 x 5 -^ 2 = 7£ 

19. Win lbs. x Wd in ft. -5- P in lbs. = Pd in ft. 

800 x 4 4- 150 = 21i 

20. TTin lbs. x Wd in ft. -*■ P<Z in ft. = Pin lbs. 

1000 x i -s- 3 = 166| 

21. »; = distance of heavier boy from fulcrum, or Pd. 

12 — x = distance of lighter boy from fulcrum, or Wd. 
90 x = GO (12 — x) JB = 4|ft. 

150 x = 720. 12 — x = 7$ ft. 

22. A door-key is a lever of the First and Second Classes combined. 
23-4. If the. screw be held vertically, the thread of a right-hand 

screw slants obliquely downward from right to left ; of a left-hand 
screw, from left to right. 

25. 2240 lbs., W, -f- 7, no. of parts of cord, = 320 lbs. P. 

2G. The number of threads to the inch depends upon the diameter of 
the pencil and the distance that the edge of the paper overlaps. 



CHAPTER II. 

Experiment 16, page 31. 

The height of the mixture is less than the height of the 
two measures of water first put into the tube. 

This is due to the fact that the air in the interstices 



A KEY TO SHAWS PHYSICS BY EXPERIMENT. 7 

between the molecules of water is driven off, and molecules 
settle into those places. 

Page 37. 

1-5. See pp. 32, 34, 35. 

6. A piece of oak is stronger than a piece of pine, because the cohe- 
sion in oak is stronger. 

Questions and Problems, page 43. 

1. See page 38. 

2. Both the ball and the slab are elastic and yield slightly 
I with the force of percussion. 

Fig. 2. 3. See page 31. 

4. Self-evident. 

5. Antimony lacks the property of malleability. 

6-7. General Properties of Matter. 

Magnitude, possessed by sand, smoke, air. 

Impenetrability, " " oil, gas, tin. 

Divisibility, " " flint, wax, glue. 

Indestructibility, " " gossamer, gunpowder, ether. 

Porosity, " " porcelain, diamond, aluminum. 

Compressibility, " " hydrogen, mercury, iron. 

Elasticity, " " mercury, tallow, putty. 

Specific Properties of Matter. 
Tenacity, possessed by tar, hemp, brass. 
Hardness, " " diamond, zinc, quartz. 

Ductility, " " glass, brass, silver. 

Brittleness, " " resin, pottery, bone. 

Malleability, " " most metals. 

Experiment 33, page 44. 

When the first needle is laid upon the surface of the 
water, it floats. This is because its weight is not sufficient 
to overcome the force of cohesion between the mole- 
cules and press them aside. When, however, the needle is 
let fall point downward, its whole weight is brought to 
bear upon one spot, and the molecules are forced apart. 



A KEY TO SHAWS PHYSICS BY EXPERIMENT. 



The needles are held in position because the surface 
tension between the two needles, tending to draw them 
tog-ether, is just equal to the surface tension on the outside, 
tending to draw them apart. The alcohol breaks the sur- 
face tension between them, and the surface tension from the 
outside pulls them apart. 

Experiment 37, page 48. 

The colored brine rises in the test-tube by the capillary 
attraction in the small spaces between the grains of salt. 

Colored water would dissolve the salt instead of rising in 
the test-tube. 

Questions, page 49. 

1. See the Summary, page 48. 

2. A liquid differs from a gas in the relation between the attractive 
and repellent forces between the molecules. In a liquid the attractive 
force is the greater, in a gas the repellent force. 

3. Gases may be reduced to a liquid condition 
by pressure and cold. 

4. The oil is drawn up the wick by capillary 
attraction. 

5. A capillary tube is any very fine tube. 

6. See page 48. 
7-8. Due to capillarity. 

9. a b, Fig. 3, = intersection of surface of water 
and nearer plate. 

10. A drop of water upon the tablecloth spreads 
rapidly. Kerosene oil is very susceptible to capillary attraction. The 
flow of sap in trees is due partly to capillary attraction. 




CHAPTEE III. 

Problems, page 53. 

1-2. Solution given. 

3. At 1,000 miles below the surface the distance has decreased £. 
£ of 500 lbs. = 375 lbs., Ana. So 3,000 mi. = f of 4,000 mi. * of 
500 lbs. = 125 lbs., Ana. 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 



9 



4. 4,000 mi. + 4,000 mi. = 8,000 mi. from the centre. 8,000 = 2 x 
4,000. (i) 2 , or h of 50 lbs. = 12$ lbs., Arts. 

5. 16.000 mi. + 4,000 mi. = 20,000 mi. from the centre. 20,000 = 
5 x 4,000. (i) 2 , or &, of "20 lbs. = f lbs., ^s. 

6. 2,000 mi. + 4,000 mi. = 6,000 mi. from the centre. 6,000 = f 
x 4,000. {ff, orf, of 1,000 tons, = 444± tons., Ans. 

Page 57. 

1. A sphere floating in water is in neutral equilibrium. 

2. The board is in stable equilibrium. 

3. The board is in unstable equilibrium. 

4. The larger the base of the inkstand, the lower is the centre of 
gravity and the more stable is the equilibrium. 



Stable. 



Unstable. 



Stable. 



Unstable. 




Stable. 




Neutral. 



8. The line of direction of the Tower of Pisa falls within the base. 

9. The weight of the arm, when thrown out, moves the centre of 
gravity farther from the pail and causes the line of direction to fall 
farther within the base. 

10. A cylinder can be placed in stable and in neutral equilibrium. 



.4t 



10 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

11. The load of stone is the more stable, the centre of gravity being 
nearer the base. 

12. When standing in the bottom of the boat, the centre of gravity 
is brought nearer the base. 

13. A pyramid is a stable structure, because its broadest section is at 
the base and its centre of gravity is near the base. 

Page GO. 

1. The motion of a body is accelerated when its velocity increases. 

2. The motion of a body is uniformly accelerated when its 
velocity increases by a constant quantity in each successive 
interval of time. 

3. Examples of retarded motion arc seen in a railway train 
coming to a stop, a rifle-ball when it is nearly spent, and a 
marble rolled across the floor. 

4. The motion of a body is uniformly retarded when its 
velocity decreases by a constant quantity in each successive 
interval of time. 

5. The motion of the ball is uniformly retarded. 
8. As much work was done in the one instance as in the 

other, viz., 16.000 ft. lbs. 

J). I'll, W in lbs., x 42, velocity in feet, = 525, momentum 
in lbs. 

Page 61. 

1. 2 x 32 = 64, distance in feet that the body would be 
carried by velocity alone. 

2. See Fig. 10. 

3. 7' 2 (no. of seconds squared) x 16 ft. = 784 ft.. Ana. 
7 x 2 — 1 = 13. 13 x 16 ft. = 208 ft.. Ans. 

\-x\4Zf ^' ^^' ve l° c it.V i» ft., -T- 32 — 14, no. of seconds. 

5. Starting with a velocity of 253 ft. the arrow will rise to 
just that height from which it must fall to acquire a velocity 
of 256 ft. 256 -4-32 = 8, no. of seconds of ascent. 8'~ x 16 ft. = 
1024 ft., height to which it will rise. 

6. In the whole time. 5 sees., the first stone has fallen 400 ft. In 2 
sees., the second stone falls 64 ft. 400 ft. — 64 ft. = 330 ft. apart, 
A>is. 



A KEY TO SHAW S PHYSICS BY EXPERIMENT. 
Page 66. 



11 




HO' Us. ' 





$«&*,% in^toH* 



Experiment 50. 

The pendulum moves from A to B by the force of gravi- 
tation. It does not stop at B, because of its inertia and tin- 
momentum it has acquired, which is sufficient to overcome 
the gravitation until the pendulum reaches C. The motion 



12 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

from A to B is uniformly accelerated ; from B to C uni- 
formly retarded. The pendulum finally comes to 'rest 
because the force imparted to it is at length expended in 
overcoming the resistance of the air. 

Experiment 51. 

The pendulums make the same number of vibrations in 
a minute. A change in the weight at the end does not 
attect the time of vibration. 

Questions and Problems, page 70. 

1. See Experiment 50. 

2. See page 67. 

3. The length is 39.1 inches. 

4. A pendulum at New York, to vibrate once in two seconds must be 
four times 39. 1 inches, or 156.4 inches. 

• 5. If a clock loses time, its pendulum is too long 

3, respect ivel" °' Vibrati ° n ™ " ^ t0 ** « ™ 4 to 6 ' « - » to 
Page 73. 

e„^ and ° the PendUlUm haS POte ' ltia ' ener »> at B ft h »* kinetic 
Page 74. 

When the stone has passed over one-third of the entire distance of 

SST one " third of its — is k — anJ *-£T- 

Questions and Problems, page 77. 

1. One's weight would be doubled. 

2. They exert one-fourth as much attraction toward each other 
2< = *• 

3. 4,000 mi. + 4,000 ,„i., distance from centre to surface - 8 000 
mi. , distance of the body from centre of earl 1, ~ ' 

8,000 = 3 x 4,000. (i )*, or 4, of 4,000 lbs. = 1,000 lbs l„, 




A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 13 

4. The distances moved over are inversely proportional to the masses. 
150 lbs. : 450 lbs. : : 1 : 3, or £ : f. 

£ of 10 mi. = 2-k mi., distance larger body would move, 
f of 10 mi. = 7| mi., distance smaller body would move. 

5. The velocity per second is 150 ft. 4- 3 t= 50 ft. 

6. The round body is always in neutral 
equilibrium ; the square body is sometimes 
in stable equilibrium. 

7. The ball is in unstable equilibrium 
because the line of direction falls outside 
of the base. See Fig. 14. 

8. 4/16 : 4/36 : : 4 : 6. 

9. 4* = 16. 16 x 16 ft. = 256 ft. high. 

10. 12 x 800 = 9,600 lbs., momentum of first ball. 
15 x 500 — 7,500 lbs., momentum of second ball. 

11. The steamboat continues to move because of its inertia and 
momentum. 

12. 9 2 x 16 ft. = 1,296 ft., Ans. 

13. Two plumb lines are not exactly parallel, because, if prolonged, 
they would meet at the centre of the earth. 

14. 3 x 2 x 16 ft. = 96 ft. velocity. 240 lbs. momentum. 

15. The earth must be removed V^ x 93,000,000 miles, or 279,000,000 
miles from the sun. 

16. The centre of gravity of the ring is in space in the centre of the 
curve. 

17. The elder pith is very light, the centre of gravity is within 
the lead, and the witch, when laid on its side, is in unstable equilib- 
rium. 

18. See Experiment 50 and explanation. A pendulum without fric- 
tion and in a perfect vacuum would never cease to vibrate. 

19. 9 x 2 - 1 = 17. 17 x 16 ft. = 272 ft. in the ninth second. 
9 2 x 16 ft. == 1,296 ft. in 9 seconds. 

20. 1,500 mi. = § of 4,000 mi. The body is f as far from the centre 
as it would be at the surface, f of 500 lbs. = 312.5 lbs., Ans. 

21. 352 -T- 32 = 11, no. of seconds of ascent. II 2 x 16 ft. = 1.936 
ft. high. Before the ball reaches the ground, 22 seconds will elapse. 

22-23. See the Summary, page 78. 

24. The bird could not move, because it would have no matter upon 
which to exert the pressure of its wings, in order to overcome the 
inertia of its body. 



14 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 



25. 150 x 140 = 21,000 ft. lbs. of work. 

ae. twv> = K E i^xg = 866J tt lbs 

27. A velocity of a mile a minute is 88 feet a second. 

K E = L (160,QOO) x 88^ = 19)360)000 ft . lbs< 

28. \ (2,000,000) x 720 2 _ 16 900,000,000 ft. lbs., KB of 1st boat 

32 

i (2,000,000) x 815" 2 OA _ ,, 01 OKn f . t n trp , aJ1 . 
i__^ ! : — 20, To ,,03 1,250 it. lbs.. A 7^ of 2d boat. 

32 -i : : 

Total energy expended = 36,957,031,250 ft. lbs. 

29. The earth should rotate more than seventeen times as fast as 
now. See foot note, page 53. 



CHAPTEE IV. 
Experiment 61, page 84. 

If we press down on the surface of the water in tube B 
with a force of twelve pounds, the water in each of the 
tubes C and D will exert an upward pressure of twelve 
pounds, because water transmits pressure equally in all 
directions. But this is true only on condition that the 
tubes C and D are of exactly the same cross section as tube 
B. For when liquids are subjected to pressure, the 
amount of pressure received upon any part of the surface 
restraining the liquid is in direct proportion to the area of 
that surface. So, as the surface of the water in E has twice 
the area of the surface of the water in B, it will exert an 
upward pressure of ten pounds, when there is a downward 
pressure of five pounds on the surface of B. The water 
in E will rise only half the distance that the stopper B is 
pushed down. 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 15 



Questions, page 88. 

1. 3 ft. x 4 ft. x %\ ft. = 30 cu. ft., contents of tank. 
30 x (32| lbs., weight of one cu. ft. of water, = 1875 lbs. 

2. 78 in. = 6| ft. 5 sq. ft. x 6£ ft. = 32£ cu. ft., contents of tank. 
32£ x 62* lbs. = 2031i lbs. 

3. The pressure upon the base of a cubical vessel is at once the 
weight of the liquid. Since, in finding the pressure upon the side of 
the vessel, only half the depth of the liquid is used, the pressure upon 
the side equals one-half the weight of the liquid. 

4. No. of sides, x \ weight of liquid = 2 times the weight. Hence, 
with the pressure upon the base, the total pressure upon a cubical tank 
is three times the weight of the liquid. 

4. 30 ft. x 26 ft. = 780 sq. ft. , surface pressed upon. 
780 sq. ft. x 13 ft., depth to middle point of surface pressed upon, 
= 10,140 cu. ft. 10,140 x 62^ lbs. = 633,750 lbs., Ans. 

Experiment 66, page 92. 

The pressure upon the rubber of the Cartesian diver is 
transmitted to the water. The water, being practically 
incompressible, is displaced, and so forced into the smaller 
bottle, compressing the air there. The smaller bottle, being 
made heavier by the added water, sinks. 

Experiment 67, page 93. 

The amount of pressure at Q and at P is exactly the 
same ; and if Q were closed, the pressure there exerted 
would counterbalance the pressure at P. But as there is 
nothing at Q to receive this pressure, the water flows with- 
out opposition ; and in seeking a similar outlet at P it 
exerts its force against the side of the tube and moves the 
mill. 

Questions and Problems, pages 94, 95. 

1. Liquids at rest exert pressure in all directions. At the upper 
surface, however, they exert no pressure upward. 



16 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

2. The pressure of a liquid upon the bottom of a vessel depends (1) 
upon the area of the surface pressed upon, (2) upon the depth of the 
liquid, (3) upon its specific gravity. 

3. A piece of floating wood displaces a quantity of water equal in 
weight to itself. 

4. See the Summary, page 94. 

5. A cubic foot of distilled water weighs 62| lbs. 

6. The water rushes up the chimney for the same reason that water 
in communicating pipes will rise as high as its source. 

8. While the stone is being lifted through the water, the weight of a 
quantity of water equal in volume to the stone is buoying it up. 

9. The weight of the water displaced by the human body, and hence 
the upward pressure, is a little greater than the weight of the human 
body. 

10. See page 84. 

11. An iron ship floats, because the hull of the ship being hollow and 
filled with air, the quantity of water equal in volume to the hull weighs 
more than the ship. 

12. 1 ft., depth of tank, + 3 ft., length of pipe, = 4 ft., or 48 ins., 
total height of column of water. 

48 x 2, number of sq. ins. in base of column, = 90 cu. in. 
T s| 8 x 62£ lbs. = 3;H lbs., Am. 

13. An equal volume of water weighs 15 lbs. — 10 lbs., or 5 lbs. 
The specific gravity is L ^, or 3. 

14. Iron floats in melted copper. 

15. The following solids will float on mercury: lead, silver, copper, 
brass, iron, tin, zinc, marble, etc. 

10. A person floats more easily in salt water, because of its greater 
specific gravity. 

17. Spec. grav. of the ore = £$, or 1|. 

18. 40 oz. x .88 = 35.2 oz., weight of a quart <>f petroleum. 
383 U.S.. or 010 oz. -v- 35.2 = 17.5, no. of quarts, or 4.375 gals. 

19. The specific gravity of sea water being greater than that of fresh 
water, the vessel displaces more water in the river, and therefore -inks 
deeper. 

20. 25 ft. x 10 ft. = 250 sq. ft., surface pressed upon. 

200 sq. ft. x 5 ft,, depth to middle point of surface, = 1250 cu. ft. 
1250 x 02* lbs. = 78,125 lbs., Am. 



A KEY TO SHAW'S PHYSICS BY EXPEKIMENT. 17 

CHAPTER V. 

Page 99. 

In order to perform Experiment 71 with water, the tube should be 
13.6 x 30 ins., or 34 ft., long. 

If the mercury stands at 28 ins., in order to perform this experiment 
with water the tube should be 13.6 x 28 ins... or 31.733 ft., long. 

Experiment 72. 

When the air is withdrawn from the tube, the pressure 
of the atmosphere is removed from the surface of the water 
within the tube, and the atmospheric pressure upon the 
surface of the water in the tumbler forces it up the tube. 

If all the air could be exhausted from a tube 36 ft. 
long, the water would rise 13.6 (see page 99) times 30 
ins., or 408 ins., or 34 ft. 

Experiment 74, page 102. 

As the water rises in the air-chamber of the force-pump, 
it compresses the air. This, in expanding,, forces the water 
out, and continues so to do while the piston is being raised 
for another stroke, when the air is again compressed. Thus 
a continuous stream of water is produced. 

The Siphon, page 105. 

The greater the difference between the levels of the liquid in the two 
vessels, the greater is the difference between the weights of the columns 
A B and C D, and hence the greater is the difference between the net 
upward pressures at A and D, and the more rapid will be the flow of 
water. 

The size of the tube makes only a slight difference in the velocity 
with which the liquid flows, due wholly to the difference in the amount 
of friction offered by the sides of the tube. In a larger tube the flow 
will be a little more rapid. Aside from this it is evident that ;i large 



18 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

tube will empty a vessel sooner than a small one because of its greater 
capacity. 

The height of the shorter arm is limited to 34 ft., because water 
cannot be raised by the atmospheric pressure above that height. 
When the liquid is at the same level in the two vessels, the flow stop-, 
because the columns A B and C D then exert the same downward 
pressure, and therefore the net upward pressure at A is no greater than 
at D. 

Experiment 78, page 108. 

The difference between the readings of the shorter tube 
and the longer tube should be the same as the reading of 
the barometer on the day of the experiment. When the 
air in the shorter tube is compressed to half its original 
volume, it is under a pressure of two atmospheres ; when it 
occupies twice the original space, it is under half an atmos- 
phere pressure. 

Questions and Problems, pages 1 10, 111. 

1. The elder pop-gun works upon the principle of the compressibility 
and elasticity of the air. 

2. See Experiment 70. 

3. The siphon with the long arm of 3 ft. will empty the vessel the 
quicker ; for, as a column of water 3 ft. long is heavier than a column 
18 ins. in length, the ultimate difference between the upward pressure 
in the longer and shorter arms of the longer siphon is greater than this 
difference of pressure in the shorter siphon. 

4. Evident from the elasticity of air and carbon dioxide. 

5. A valve is a tightly closing lid, usually made so as to open in only 
one direction. 

6. 30 ins. : 27 ins. :: 14.7 lbs. : 13.23 lbs.. Ans. 

7. The water can be pumped out of any well where the perpendicular 
distance from the surface of the water to the piston when raised is 
somewhat less than 34 ft. 

8. 7 x 000 ft. (see page 100) = G,300 ft., height of mountain. 
30 ins. : 22 ins. :: 14.7 lbs. : 10.78 lbs. pressure, 

9. The paper seals the glass, preventing the entrance of the air at 
any one point, and allowing the atmospheric pressure to sustain the 
water. 



A KEY TO SHAW'S PHYSICS BY EXPEEIMENT. 19 

10. A vacuum is space devoid of all matter. 

11. Water may be raised out of a well 40 ft. deep by using a pump 
with a plunger of such a length that, when it is raised, the piston-head 
shall be somewhat less than 34 ft. above the surface of the water in the 
well. 

12. Left for the student. 

13. Theoretically the air could sustain a column of water 13.6 (see 
page 99) x 28 ins., or 31.73 ft., in height ; but because a perfect vacuum 
cannot be made by an ordinary pump, and because the construction of 
the pump demands that the water shall rise a few inches above the 
lower valve, the valve must be placed somewhat less than 31.73 ft. 
above the surface of the water in the well. 

14. Air not only exerts pressure downward, but it transmits that 
pressure in all directions. 

15. The pressure exerted upon the air by the closing door is trans- 
mitted through the adjoining room to the remote door. Or, a different 
case, the closing door removes the atmospheric pressure from the inner 
side of the remote door, and the atmospheric pressure on its outer side 
closes it. 

17. The gas will occupy half its original volume, or half a cubic foot. 



CHAPTER VI. 

Questions and Problems, page 119. 

From Fig. 103 it is evident that there are j§§ or f as many degrees 
between two temperatures on the Fahrenheit scale as on the Centigrade, 
and f as many on the Centigrade scale as on the Fahrenheit. Hence, 
to reduce from Centigrade to Fahrenheit, multiply by f and add 32°, 
because 0° C. = 32^ Fahr. And to reduce from Fahrenheit to Centi- 
grade, subtract 32° and multiply by tj . 

1. | of 72° = 129.6° in Fahrenheit degrees. 

2. 60° - 32° = 28°. | of 28° = 15|° C. 

3. Answers : 10° C, 1H° C, - 40° C, 93^° C, 7£° C. 

4. § of 80° = 144°. 144° + 32° = 176° Fahr. 
Answers: 59° Fahr., 41° Fahr.. — | c Fahr. 

5. Answers: 48|° C, 176° Fahr., 17^ r C, - f° Fahr., 65f° C, 
-35jj°C., 37|°C. 



20 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 



Experiment 92. 

The gauze conducts away the heat of the flame so rapidly 
that the gas on the side of the gauze opposite the flame 
cannot become heated sufficiently to ignite. 

Page 129. 

When water boils, all the air is expelled from the vessel containing 
the water, and its place filled by steam. There being, therefore, 
nothing to impede the motion of the water as it is thrown about, it 
strikes directly against itself and against the glass without the usual 
intervening cushion of air, and so produces a clicking sound. What- 
ever cushion of steam there may be is instantly dissolved. 

Experiment 98. 

A rise in temperature, an increase in the extent of sur- 
face, and removal of pressure of the atmosphere, all increase 
the evaporation. 

Experiment 101. 

Should it be found difficult to obtain satisfactory results 
with Experiment 101, the following may be substituted : 

Put half a pound of iron filings into a beaker and sus- 
pend the beaker for some minutes in a vessel of water, 
which is to be kept boiling, stirring the filings meanwhile 
with the bulb of a thermometer. Weigh out half a pound 
of water of ordinary temperature in another beaker, and set 
this on a number of thicknesses of cloth or paper, to pre- 
vent contact with the table. When the temperature of the 
filings has ceased to change, note it ; and then, after the 
thermometer has been allowed to cool, note the temperature 
of the water in the other beaker. Now remove the beaker 
of filings from the vessel of boiling water and pour them 
into the beaker of water, stirring constantly with the 
thermometer. When the temperature of the mixture has 



A KEY TO SHAW'S PHYSICS BY EXPEKIMENT. 21 

ceased to change, note it. It will be found that the water 
has risen in temperature and the iron has fallen, to the 
temperature of the mixture. The one has evidently gained 
as much heat as the other has lost, but the changes in tem- 
perature have been different. By dividing the number of 
degrees of change in the water by the number of degrees 
of change in the iron, the specific heat of the iron may be 
found. Error may be prevented by having the temperature 
of the thermometer that expected in the mixture before 
stirring the mixture. 

Experiment 102, page 133. 

The ether, in evaporating, takes the heat from that with 
which it is in contact ; first from the cotton ; the cotton 
takes it from the glass ; the.glass takes it from the mercury. 
This contracts, and a lower reading is seen on the ther- 
mometer. 

Experiment 103, page 133, 

AVe are unable to form ice upon the sheet of copper as 
upon the wood ; for as soon as any part of the copper is 
cooled, heat is immediately conducted thither through the 
surrounding metal. 

Questions and Problems, pages 142-145. 

1. See the Summary, page 141. 

2. Cold is the absence of heat. 

3. Ice has a low degree of heat. 

4. 5, 6. See the Summary, page 141. 

7. A thermometer is an instrument for measuring temperature; a 
barometer, for measuring atmospheric pressure. 

8. See explanation to the problems on page 19 of the Key. 

- 40° C. x | + 32 c = - 40° Fahr. . • . - 40° Pahr. = - 40 : C. 

9. | (-36° -32° ) = -37§° C. 

10. Answer: — 6§° C. 

11. Answer : 68° Fahr. 



22 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

12. The bore of a thermometer should be uniform throughout ; for 
as the degrees are always of equal length, it would otherwise, not indi- 
cate temperature correctly. 

13. Heat cannot be passed from a colder to a hotter body. 

14. Heat usually increases the size of an object. 

15. Ice, cast iron, bismuth, antimony are exceptions to the fore- 
going rule. 

10. The human body takes note of only comparative temperatures. 

17. Evident from the answer of No. 1G. 

18. For measuring the highest temperatures an instrument called a 
pyrometer is used. Formerly it was simply a thermometer made of 
hard metal, but the apparatus now employed is based upon the phe- 
nomena of thermo-electricity. 

19. For medium and low temperatures a mercurial thermometer will 
answer. 

20. For extremely low temperatures alcohol is best. It has recently 
been solidified at —130.5° C. 

21. When the axles of coach-wheels are not greased, they expand 
with the heat caused by the friction to such a degree as to stop the 
wheel. 

22. Because the rails contract in winter and widen the spaces. 

23. Wagon-tires are put on wheels when hot in order that, as the 
tires contract with cooling, they may bind the parts of the wheel 
tightly together. 

24. The fixed points of a mercurial thermometer are the zero point 
and the temperature at which distilled water boils. 

25. Glass is a poor conductor of heat. W T hen suddenly healed, 
therefore, it expands irregularly. This tends to bend the glass, and, 
being brittle, it breaks. 

26. See Experiment 104. 

27. See Experiment 91. 

28. 29. The warm carbon dioxide given off from the lungs is lighter 
than the surrounding air, therefore the bubble ascends. But this gas 
when cold is heavier than air ; hence the bubble soon descends 
again. 

30. From the preceding answer we should conclude that the bad air 
of a room is near the ceiling. For ventilation, therefore, two openings 
should be provided ; one near the ceiling, as an exit for the bad air. 
the other lower down, to admit the pure air. 

31. There being a greater difference in density between the outside 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 23 

air and that immediately above the fire on a cold day than there is on 
a warm day, a better draught is caused. 
32, 33. Left for the student. 

34. When a fire is first lighted, there is often not sufficient heat to 
cause a draught, the air in the chimney does not rise, and the smoke 
comes into the room. 

35. When the hand is laid on a piece of cold iron, the heat imparted 
to the metal is conducted immediately away. The hand thus becomes 
cooled, and we feel the sensation. Wood, however, is a poor conductor, 
and remains warm beneath the hand. 

36. Evident from the preceding answer. 

37. Flannel is a poor conductor of heat. 

38. The carpet is not so good a conductor as the marble, and hence 
feels warmer. 

39. If both were warmer than the body, the marble would seem to 
have the higher temperature. 

40. Linen and cotton sheets are better conductors than woollen blan- 
kets. 

41. The glass of the thermometer is not an absolute non-con- 
ductor. 

42. Glass has the property of allowing luminous rays of radiant heat 
to pass through it, but not dark, or obscure, rays. Hence the sun's 
rays readily enter the hot-bed, but the obscure rays of heat reflected 
and radiated from the earth cannot pass beyond the glass, and expend 
themselves upon the air beneath. All around the hot-bed. however, 
the obscure rays from the earth reach to a greater elevation, and their 
heat, being communicated to a greater depth of atmosphere, does not 
raise its temperature so high. 

43. Double windows prevent heat from escaping, because the air 
confined between the two glasses acts as a non-conductor. Further- 
more, when there is only one thickness of glass, as soon as heat from the 
room has been communicated through the glass to the air outside, this 
warm air moves away, and thus there is always cold air in contact with 
the glass, which cools the room. By a double window the air warmed 
from the room is imprisoned, and the cold air cannot come in contact 
with the glass next to the room. 

44. The covering of snow prevents radiation from the earth. 

45. The large quantity of air in the snow renders snow a poor con- 
ductor. 

46. Heat increases evaporation. 



24 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

47. Bright pans do not absorb the heal of the oven as readily as dark 
ones. 

48. The rapidity of evaporation is affected by temperature, extent 
of surface, atmospheric pressure, and the amount of moisture in the 
atmosphere. 

49. Water will boil at a lower temperature on the lop of a mountain 
than at the sea level, because the atmospheric pressure is Less. 

50. The boiling-point of a liquid is affected, by diminished pressure, 
increased pressure, and the amount of foreign matter present. 

51. See the Summary, page 142. 

52. See answer for page 129, on page 20 of the Key. 

53. The steam rising through the water and escaping at the surface 
causes boiling water to bubble. 

54. Distillation depends upon the principle that by means of heat a 
liquid or solid may be expanded into a gas. and by means of cold this 
gas may be again condensed into the liquid or solid state, as the case 
may be. 

55. Latent heat can be changed into sensible heat. 

50. The current of air from the fan evaporates the insensible perspi- 
ration. 

57. The cold pitcher condenses the moisture in the atmosphere «>f 
the room. 

58. Salt causes ice to melt because of the affinity of the -.alt for water. 
In melting, the ice takes the sensible heat from objects around it. to be 
turned into latent heat, and thus renders surrounding bodies cold. 

59. The steam is condensed on the cold tumbler. 

60. The watery vapor in the breath is condensed. 

61. By the maximum density of water we understand that condition 
when the molecules are closest together, or when a given aim unit 
occupies the least possible space. Water reaches ii> maximum density 
at 39.2° Fahr. 

62. As water expands in freezing, the specific gravity of ice is less 
than that of water at ordinary temperature. 

63. As water freezes at32° and reaches its maximum density at 39.2°, 
it must occupy more space when frozen. 

64. Heat may be diffused by conduction, by convection, by radiation. 

65. The earth's surface is heated by radiation from the sun. 

66. The atmosphere is heated by radiation from the sun and from 
the earth, and by convection as it comes in contact with the warm 
earth. 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 25 

t'»7. A good absorbenl is not a good reflector. 

• 68, There is no reason why a good conductor should or should not 
be a good radiator. The conditions that make the one are entirely 
different from those that make the other. 

• '.!). After the water reaches 212° all heat is expended in changing 
the water to steam. 

70. Boiling- is noisy, evaporation is quiet ; boiling takes place within 
the body of the liquid, evaporation takes place at the surface. 

71. The water on the floor evaporates. In so doing it changes the 
sensible heat of the atmosphere to latent heat within itself, and the 
atmosphere becomes cooled. 

72. The water was caused to circulate by the cold. The circulation 
stopped when the water reached the temperature 39.2° Fahr. 



CHAP TEE VII. 

Experiment 131, page 162, 

Because of the extreme lightness of the lycopodium, the 
currents of air produced by the vibrations draw it away 
from the nodes. 

Questions and Problems, pages 169-171. 

1. Vibration is motion back and forth. Amplitude of vibration is 
the distance a body moves from the farthest point on one side of its 
position of rest to the farthest point on the other side. 

2. See the Summary, page 167. 

3. The particles in a water-wave move up and down, or at right- 
angles to the direction taken by the wave. In a sound-wave the parti- 
cles of air move back and forth, or along lines parallel to the direction 
taken by their respective parts of the wave. 

4. A wave length includes one complete crest and one complete 
trough, or it includes one complete condensation and one complete 
rarefaction. 

5. See the Summary, page 168. 

6. 27 ft. -r- 16 = 1]^ ft., average length of waves. 

7. 1,120 ft. -v- 560 = 2 ft., wave length. 

8. 1,120 ft., velocity of sound, x 4, no. of seconds, = 4,480ft. distant. 



26 



A KEY TO SHAW S PHYSICS BY EXPERIMENT. 



9. See Experiment 129. 

10. 1,120 ft. h- 512 = 2-j 3 6 - ft., length of sound wave. 

11. A sound wave is spherical in form. 

12. Sound travels less rapidly in air than in liquids and solids. 

13. The larger string will produce a tone of lower pitch. 

14. The longer string will produce the lower tone. 

15. The string subjected to the more tension will produce a tone of 
higher pitch than the other. 

16. See page 151. 

17. This question cannot be answered by saying that, as water is a 
good conductor of sound, a mixture of water and air will convey sound 
with greater readiness than air alone. For the presence of watery 
vapor in the atmosphere tends to diminish rather than to increase its 
conductivity. According to Tyndall, the explanation is that on some 
days, apparently clear, there are invisible stria 1 of aqueous vapor in I he 
atmosphere, which intercept the passage of the sound-waves. If there 
be no such striae, however, sounds will travel to much greater distances 
even in a thick haze. Further, in foggy weather, when the air is per- 
fectly still and the water calm, the sound-waves travel more readily 
than when they are broken by rough weather. 

18. Left for the student, 

19, 20. See the Summary, page 168. 

21. An echo is sound heard after reflection. 

22. \ of 20 sees. = 10 sees., time required for the sound 
to travel one way, from the rocks to the observer. 
1,090 ft., velocity of sound, x 10=10,900 ft, or 2.06 
miles, distance of rocks. 

Or, 1,120 ft., velocity of sound, x 10 = 11,200 ft., or 2.12 
miles. 

23. The intensity of sound heard by the first observer is 
to the intensity of sound heard by the second observer as 
2- distance of the second observer, is to (if distance of the 
first observer, or as 4 is to -^, or, reducing, as 64 is to 1. 

24. Left for the student. 

„ " 25. "Musical sound is that which produces a continu- 

Fig. 15. * 

ous sensation, and the musical value of which can be 

estimated ; while noise is either a sound of too short a duration to 

be determined, or else is a confused mixture of many discordant 

sounds." — Gemot. 

26. 126 x 2 = 252 vibrations, Ana. 




A KEY TO SHAW S PHYSICS BY EXPERIMENT. Z / 

27. See page 160. 

28. The middle G has 264 vibrations per second. 
1,120 ft. -4- 264 = 4 T3 % ft., wave length. 

29. 30. See the Summary, page 168, and Figs. 131, 138. 
31-33. See the Summary, pages 168-9. 

34. A tuning-fork has a node on each side by the handle: and unless 
it be a perfect one. it will show nodes in other places. They may be 
located by a bead suspended by a thread. (See Fig. 15.) 

35. The harder one blows, the greater is the speed with which the 
disk revolves, and therefore the greater is the number of vibrations 
per second. 

CHAPTEE VIII. 
Experiment 140, page 174. 

There are two light shadows, because the shadow cast 
from each candle is in part lighted by the other candle. 
But where these two shadows overlap, light is received 
from neither candle, and a dark shadow is the result. 



Constructions, page 184. 
£ c d 




a m o, angle of incidence, = o m e, angle of reflection. 
b n p, angle of incidence, = p n d, angle of reflection, and so on. 



28 



A KEY TO SHAW S PHYSICS BY EXPERIMENT. 



Experiment 153. 

A convex mirror forms a virtual image. 




Questions, pages 211, 212. 

1. Luminous bodies are bodies that emit light. 

2. See page 172. 

3. A shadow is a portion of space from which light is excluded by an 
intervening body. 

4. The umbra is formed by the exclusion of all the light, the 
penumbra by the exclusion of only part of the light. 

5. A part of the luminous body can be seen from the penumbra. 

6. The intensity of light varies inversely as the square of the dis- 
tance from the source of 
light. 

7. The images formed by 
means of small apertures 
are inverted. For, as the 
rays of light from the ob- 
ject 0, in order to reach the 
screen &,must pass through 
the aperture A, they must there cross, rendering the image inverted. 

8. M = mirror. 

OP — perpendicular. ; 

A = incident ray. 

B = reflected ray. 
A P = angle of incidence. 
B P = angle of reflection. 

9. A real image can be thrown upon 
screen, a virtual image cannot. 

10. See Fig. 154. 

11. One image is seen by reflection from the back of the mirror, ami 
a second is seen by reflection from the front of the glass. Other 
images, sometimes above, sometimes below these, are formed by rays 
of light which, coming from the object and reflected by the back of the 
mirror, undergo total reflection at the front surface of the glass, and 
afterwards a second or even a third and fourth reflection from the 
back of the mirror, before reaching the eye. The image formed by 
the back of the mirror is the brightest, because of the superior reflect- 
ing power of mercury. 






Fig. 19. 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 29 

12. In diffusion of light the rays are scattered; in reflection they 
bear a definite relation; by absorption they disappear within the ab- 
sorbing substance. 

13. The image formed by a plane mirror is of the size of the object, 
and just as far behind the mirror as the object is in front of it. 

14. If a ray of sunlight strikes the side of a prism at a sufficiently 
small angle, it will be reflected instead of entering the prism. 

15. 360° -h 30° = 12. 12 — 1 = 11, no. of images. 
36(T -f- 120° =3. 3 — 1 = 2, no. of images. 

16. The image formed by a convex mirror is smaller than the object. 

17. When the object is near a concave mirror [». e., not as far away 
as twice the focal length of the mirror] the image is larger than the 
object. When the object is far removed from a concave mirror [«. e., 
beyond the centre of curvature, or beyond twice the focal length] the 
image is smaller. 

18. The laws of refraction, page 188, are obeyed. 

19. See page 191. 

20. Hold the lens in the direct rays of the sun and let them come to 
a focus upon a piece of white paper. Move the paper until the spot of 
light upon it is as small as can be obtained. The distance from the 
paper to the lens will be the focal length of the lens. 

21. The band of seven colors formed by the decomposition of rays of 
sunlight is known as the solar spectrum. 

22. Violet, ultramarine blue, etc., as on page 195, only in the 
reverse order. Red is the least refracted. 

23. See the Summary, page 210. 

24. See pages 202, 203. 

25. Violet, etc , as on page 195. Red, etc., in the reverse order. 

26. Two refractions and two reflections are required to produce the 
secondary bow; two refractions and one reflection, to produce the 
primary bow. 

27. 




30 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

28. In near-sightedness the globe of the eye is too long; in far- 
sightedness it is too short. For the former concave glasses are re- 
quired; because, as the crystalline lens brings the rays to a focus 
before they reach the retina, it is too powerful, and the opposite kind 
of a lens must counteract its effect. In the latter, as the rays are not 
brought to a focus soon enough, the lens of the eye is too weak, and 
must be reenforced by a convex lens. 



CHAPTER IX. 
Experiment 179, page 220. 

When like poles are tog-ether, one reenforces the other, 
and the key is sustained. When unlike poles are together, 
the key falls. 

Questions, pages 225, 226. 

1. Lines of force are imaginary lines radiating from the poles of a 
magnet, along which its energy is exerted. 

2. The magnetic field is the space through which the influence of 
a magnet extends. 

3. By the polarity of a magnet is meant its property of exerting 
opposite tendencies at its poles. 

4. See page 217. 

5. See page 213. 

6. The neutral line of a magnet is a line extending around the mag- 
net midway between the poles, which exhibits neither powers of attrac- 
tion nor repulsion. 

7. As the lines of force extend more readily through iron than 
through the air, the armature keeps them in closer proximity to the 
magnet, and so preserves the strength of the magnet. 

8. Every molecule of a magnet possesses polarity. When, there- 
fore, a magnet is broken, each face of fracture must exhibit that 
property. 

9. See page 222. 

10. See page 223. 

11. The magnetism may be destroyed by heating the magnet. 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 31 



12, ^s± 13. 




.0. 



Fig. 21. Fig. 22. Fig. 23. 

14: The magnetic needle points approximately northward toward the 
magnetic pole. 

15. The magnetic needle does not always point in one direction, as 
the line of no variation is slowly moving, and the declination is, there- 
fore, not constant, There are, besides, other slight variations of the 
needle, known as daily and yearly variations. 

16, 17. See page 219 and note. 

Experiment 189. 

Negative electricity is developed on the sealing-wax, 
positive on the flannel cap. 

Experiment 192. 

The sealing-wax is charged with negative electricity. On 
the side of the tin next the sealing-wax there is positive 
electricity, on the opposite side there is negative. 

Law of Induction. — When an electrified body is brought 
near an unelectrified conductor there is induced on the nearer 
side of that conductor electricity of a kind opposite to that 
possessed by the electrified body, and on the farther side 
of the conductor electricity is induced of the same kind as 
that possessed by the electrified body. 

Experiment 194. 

When you touch the upper side pf c with the linger, neg- 
ative electricity passes to the earth ; and when c is lifted, 
it is charged with positive electricity. 



32 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

Experiment 196. 

See answer to question 9 of page 237. 

Questions, pages 236, 237. 

1. Bodies charged with unlike kinds of electricity attract each 
other ; bodies charged with like kinds repel each other. 

2. See Experiment 189. 

3. Substances that offer a free passage to electricity are called con- 
ductors ; those substances which do not allow electricity to flow over 
or through them are called non-conductors, or insulators. 

4. See Experiment 188. 

5. See Experiment 193 and Fig. 206. 

6. All unelectrified bodies are supposed to be covered with both posi- 
tive and negative electricity in equal amount, so that the one neutralizes, 
the other. When an electrified body is brought near, it decomposes 
this neutral electricity, attracting one kind to the nearer side of the 
unelectrified body and repelling the other to the farther side. 

7. See page 234. 

8. See Experiment 196. 

9. When the tinfoil within the jar becomes charged with posit he 
electricity from an electrophorus or other electrical machine, negative 
electricity is induced on the tinfoil without the jar. There the positive 
electricity induced, being repelled, escapes through the hand to the 
earth, thus leaving the outer surface charged with negative electricity. 
When the jar is discharged, the tinfoil without and that within the jar 
are brought into close proximity through a conductor known as a dis- 
charger. The electricity then leaps across the intervening space, and 
the electricity within and without the jar are neutralized. 

10. The Leyden jar receives and retains a number of charges of 
electricity, and when discharged presents nearly all of them in one. 

11. The student received a shock of electricity because his body 
acted as a discharger, the connection being made through the earth. 
He should have stood upon a glass plate or other insulator. 

12. Lightning-rods are pointed in order that the discharge may be 
silent. 

Experiment 207, page 254. 

One hundred feet of No. 30 copper wire offers more 
resistance than one hundred feet of No. 16 copper wire. 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 33 

One hundred feet of No. 30 copper wire offers greater 
resistance than fifty feet of the same. Fifty feet of No. 30 
German silver wire offers more resistance than fifty feet of 
Xo. 30 copper wire. 

Experiment 208. 

When the plates are near together the galvanometer 
shows the greater strength of current. 

Experiment 209. 

With the copper and zinc the needle was deflected the 
more. If we replace the copper with carbon, it will be 
deflected still more. 

Problems, page 259. 

1. Solution given. 

a _ E 8 volts 

2 ' C = iTT7 = 3o& = 2f ampereS - 

_ 15 volts 3 

3. -r^ — : — = — amperes. 
2o ohms o 

. D E 15 volts 150 iA , 

4. R + r — -^ = r— ^ = — - = 10 ohms. 

C l.o amperes lo 

5. E = C {R + r) = 7 amperes x 8 ohms = 56 volts. 

Experiment 212, page 264. 

The hydrogen is the electro-positive element, the oxygen 
the electro-negative. 

Experiment 222, page 276. 

When the magnets are withdrawn from the helix, the 
current produced is in the opposite direction to that caused 
by plunging in the magnets. When the south poles are 
plunged in, the current is in the opposite direction to that 
caused by plunging in the north poles. To give a current 
3 



34 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

in the same direction as that produced when the south 
poles are withdrawn, the north poles must be plunged in, 
and vice versa. 

Experiment 223, pages 278, 279. 

The currents produced in the secondary coil by putting 
in and taking out the primary coil agree in direction with 
the currents produced by putting in and taking out the 
corresponding poles of a bar magnet. 

As the lines of force traverse iron more readily than air, 
the presence of the iron wire inside the primary coil renders 
the lines of force greater in number, and so produces a 
stronger current in the secondary coil. 

Experiment 224, page 281. 

The current, varying with the pressure on the carbon, 
causes the strength of the magnet to vary, and so the disk 
E moves. 

Questions and Problems, pages 301-303. 

1. See page 238. Zinc should be amalgamated to prevent its being 
eaten away by the acid when the wires are disconnected. Amalga- 
mating also prevents local currents in the zinc. 

2, 3. See page 240. 

4. See page 241. 

5. In the cell the current always flows from the zinc to the other 
element ; outside the cell, in the wire, it flows to the zinc from the 
other element. 

6. The surface of the silver plate is covered with finely divided 
platinum. 

7. The essential parts of a voltaic cell are two elements, electrically 
related to each other as positive and negative, and an exciting fluid. 

8. After a gravity cell has worked some time, it will be observed that 
there are two liquids in the jar, a solution of copper sulphate, which is 
heavy and sinks to the bottom, and a solution of lighter zinc sulphate, 
which floats on the top. Hence the name. 



A KEY TO SHAW S PHYSICS BY EXPERIMENT. 65 

9. See the Summary, page 299. The parts of an electro-magnet are 
the helix and the core. 

10. The electric telegraph works upon the principle of the electro- 
magnet. 

11. See page 246. 

12. See Fig. 220 and page 248. 

13. See page 250. 

14. See page 251. The influence of the earth's magnetism upon one 
of the pair of needles counteracts the influence upon the other needle. 

15. See page 250. 

16. See the Summary, page 299. External resistance depends upon 
the length, cross-section, and material of the conductor. 

17-21. See Summary, pages 299, 300. 
22. See Figs. 231, 232, 233. 



E _ 7.5 volts 75 5 
i + •/■ " 
11 volts 1 



23. C — -5 — -jr—\ = ™ = w ampere 

R + r 9 ohms 90 6 

11 T 

24. 



33 ohms " 



25. C = ~- r . .:C(R + r) = E. .;R + r = ^. 

9 volts 90 e . 

R + r = —5 ^ — = — - r= 5 ohms. 

1.8 amperes 18 

26. C= -^—. .-. G(R + r) = E. 

It 4- r 

E — 2.2 amperes x 12 ohms = 26.4 volts. 

27. 1.5 volts, E. M. F. of, one cell, x 4. no. of cells, =6 volts. 

1.5 ohms, resistance in one cell, x 4 = 6 ohms = r. 

E 6 volts 6 3 

C = 75 = 5 — ; 7 . — ; — = — - =— ami eres. 

R + ;• 8 ohms + 6 ohms 14 t l 

28. 1.5 volts = E. M. F. of one cell, and of four cells arranged 
parallel. 

1.5 ohms, r of one cell, -f- 4, no. of cells. = .375 ohms. 

n E 1-5 volts 1.5 

C = -75 = a — t ^^ — i — = o .)-- = Aid amperes. 

R + r 8 ohms + .Sto ohms 8.0 10 l 

29. 1.5 volts x 2, no. of cells in series. = 3 volts. E. M. F. of one 
pair, and of two pairs arranged parallel. 

1.5 ohms, >■ in one cell, x 2, no. of cells in series. -=- 2, no. of pairs 
arranged parallel, =1.5 ohms. 

r _ E _ 3 volts _ 3 _ 30 _ 6 

"" R + r ~ 8 ohms + 1.5 ohms ~ 9.5 ~ 95 ~ 19 am l 3eres - 



36 A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 

30. 1.25 volts, E. M. F. of one cell, x 6, no. of cells in series, = 7.5 
volts. 

1.5 ohms, r of one cell, x 6=9 ohms, resistance in six cells arranged 
in series. 

_ E _ 7.5 volts _ 7^5 _ 75 _ 15 _ 

~ R + r~ 20 ohms + 9 ohms," 29 ~ 290 "" 58 " ' 8 Bm I» res » 
current strength when the cells are arranged in series. 

1.25 volts, E. M. F. of one cell, and of six cells arranged parallel. 
1.5 ohms, r of one cell, -s- 6 = .25 ohms, r of six cells arranged par- 
allel. 

_ E _ 1.25 volts _ 1^25 _ JL25 _ 5 _ 

R + r ~20 ohms + .25 ohms ~ 20.25 _ 2025 ~ 81 ~ ' *** 

peres, current strength when the cells are arranged parallel. 
Series is in this case the better arrangement. 

„. „ E 2 volts 2 

31. V = 73 = ^— r ; : — ; ; = -z. amperes. 

R + r 5 ohms (r not given) 5 

QO ^ 1.5 volts 15 3 . . 

32. 6=75= =— r = - — — amperes in J.. 

R o ohms oO 10 ■ 

Resistance in B : resistance in A :: cur. str. in A : cur. str. in B. 

3 5 

3 ohms : 5 ohms :: t-- amperes : —amperes. 

See page 263 for law of divided circuit. 

33. See Fig. 235, page 263. The hydrogen is given off at the nega- 
tive electrode ; the oxygen, at the positive electrode. 

34. See page 267. 

35. 36. See Experiment 215. 

37. The copper should be attached to the negative pole 
38. 



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\ 



39. The current, if strong, would tend to make the needle dip. 

40. The force, or pressure, with which electricity moves forward in 
the voltaic circuit is called electro-motive force. The quantity of elee- 



A KEY TO SHAW'S PHYSICS BY EXPERIMENT. 37 

tricity which in any unit of time traverses a section of a conductor 
determines the current strength. 

41. The number of amperes, which we have called the current 
strength, is the same at either end of the wire. The smaller end of 
the wire, if small enough, will have the higher temperature. 

Where, then, is the loss of electrical energy manifested which here 
has been transformed into heat ? Question 17 brings to notice the 
fact that amperes alone are not a measure of electrical energy. The 
mechanical effectiveness of a current depends upon both current 
strength in amperes and the E . M. F. under which the current is 
moving. It is measured in watts or voltamperes. 

A watt is the energy of an electric current of one ampere impelled 
by- an E. M. F. of one volt. In any current the number of watts, or 
voltamperes, equals the product of the volts by the amperes, or W = C E. 
This product determines the amount of work that the current can do, 
and in doing work, such as heating the wire in the case before us, 
watts are expended. One horse-power = 746 watts. 

Here it might be well to add that in any uniform conductor there is 
a constant falling off in potential, and so a decrease in E. M. F., as the 
distance from the source of the current increases. 

Further, a unit of measure for quantity of electricity is the ampere- 
hour, which is the quantity that traverses the section of a conductor 
in an hour, when (7=1 ampere. The coulomb is also in use, which is 
the quantity of electricity that passes in one second, when G ■= 1 am- 
pere. 

42. The direction of the current can be determined by means of a 
magnetic needle. See Ampere's rule, page 250. 

43. There are two causes for the delicacy of the galvanometer made 
with an astatic pair. (1) The astatic needle turns freely and without 
hindrance from the directive influence of the earth's magnetism. (2) 
The current in the wire, where it passes between the needles of the 
pair, tends to turn both needles in the same direction. 

44. 45. See Experiment 215. 

46. See page 279. 

47. By making a change in the number of lines of force that pass 
through the space enclosed by a coil of wire a current of electricity is 
induced in the coil. This change may be effected by moving toward 
or from the coil a magnet or an electrical conductor through which a 
current is flowing, or by making, breaking, or altering the strength of 
a current in a neighboring conductor. 



mm 



UBRARY OF CONGRESS 



III 



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Pt 






